As a rule I never repeat a painting. I do from time to time revisit an older successful painting, and work from it and the photographs of the original subject, to paint a new and fresh painting. I attempt to keep the essence of what I enjoyed about the original, but to re-explore new avenues, perhaps with a slightly differing perspective, new colors or adjusting the 'all important' line placement. (Example below)
Monet seemed to spend much of his life painting haystacks
However, very, very rarely I do this...
...Attempting to paint two painting produced simultaneously, exploring the same subject in basically the same way.
I do this partly as an exercise, but also to reinforce the idea that I'm just not very keen on working in this 'carbon copy' method. Despite my disinclination to work in this way, I think both paintings turned out well and had interesting aspects to them. It's enjoyable to see them side by side and spot the differences and changes between the two works.
To keep things interesting for me, the paintings were produced at different sizes and the large one is painted on a heavily textured ground, producing a very different effect.